This description relates to an approach to executing graph-based program specifications.
One approach to data flow computation makes use of a graph-based representation in which computational components corresponding to nodes (vertices) of a graph are coupled by data flows corresponding to links (directed edges) of the graph (called a “dataflow graph”). A downstream component connected to an upstream component by a data flow link receives an ordered stream of input data elements, and processes the input data elements in the received order, optionally generating one or more corresponding flows of output data elements. A system for executing such graph-based computations is described in prior U.S. Pat. No. 5,966,072, titled “EXECUTING COMPUTATIONS EXPRESSED AS GRAPHS,” incorporated herein by reference. In an implementation related to the approach described in that prior patent, each component is implemented as a process that is hosted on one of typically multiple computer servers. Each computer server may have multiple such component processes active at any one time, and an operating system (e.g., Unix) scheduler shares resources (e.g., processor time, and/or processor cores) among the components hosted on that server. In such an implementation, data flows between components may be implemented using data communication services of the operating system and data network connecting the servers (e.g., named pipes, TCP/IP sessions, etc.). A subset of the components generally serve as sources and/or sinks of data from the overall computation, for example, to and/or from data files, database tables, and external data flows. After the component processes and data flows are established, for example, by a coordinating process, data then flows through the overall computation system implementing the computation expressed as a graph generally governed by availability of input data at each component and scheduling of computing resources for each of the components. Parallelism can therefore be achieved at least by enabling different components to be executed in parallel by different processes (hosted on the same or different server computers or processor cores), where different components executing in parallel on different paths through a dataflow graph is referred to herein as component parallelism, and different components executing in parallel on different portion of the same path through a dataflow graph is referred to herein as pipeline parallelism.
Other forms of parallelism are also supported by such an approach. For example, an input data set may be partitioned, for example, according to a partition of values of a field in records of the data set, with each part being sent to a separate copy of a component that processes records of the data set. Such separate copies (or “instances”) of a component may be executed on separate server computers or separate processor cores of a server computer, thereby achieving what is referred to herein as data parallelism. The results of the separate components may be merged to again form a single data flow or data set. The number of computers or processor cores used to execute instances of the component would be designated by a developer at the time the dataflow graph is developed.
Various approaches may be used to improve efficiency of such an approach. For example, each instance of a component does not necessarily have to be hosted in its own operating system process, for example, using one operating system process to implement multiple components (e.g., components forming a connected subgraph of a larger graph).
At least some implementations of the approach described above suffer from limitations in relation to the efficiency of execution of the resulting processes on the underlying computer servers. For example, the limitations may be related to difficulty in reconfiguring a running instance of a graph to change a degree of data parallelism, to change to servers that host various components, and/or to balance load on different computation resources. Existing graph-based computation systems also suffer from slow startup times, often because too many processes are initiated unnecessarily, wasting large amounts of memory. Generally, processes start at the start-up of graph execution, and end when graph execution completes.
Other systems for distributing computation have been used in which an overall computation is divided into smaller parts, and the parts are distributed from one master computer server to various other (e.g., “slave”) computer servers, which each independently perform a computation and which return their result to a master server. Some of such approaches are referred to as “grid computing.” However, such approaches generally rely on the independence of each computation, without providing a mechanism for passing data between the computation parts, or scheduling and/or sequencing execution of the parts, except via the master computer server that invokes those parts. Therefore such approaches do not provide a direct and efficient solution to hosting computation involving interactions between multiple components.
Another approach for distributed computation on a large dataset makes use of a MapReduce framework, for example, as embodied in the Apache Hadoop® system. Generally, Hadoop has a distributed filesystem in which parts for each named file are distributed. A user specifies a computation in terms of two functions: a map function, which is executed on all the parts of the named inputs in a distributed manner, and a reduce function that is executed on parts of the output of the map function executions. The outputs of the map function executions are partitioned and stored in intermediate parts again in the distributed filesystem. The reduce function is then executed in a distributed manner to process the intermediate parts, yielding the result of the overall computation. Although computations that can be expressed in a MapReduce framework, and whose inputs and outputs are amendable for storage within the filesystem of the mapreduce framework can be executed efficiently, many computations do not match this framework and/or are not easily adapted to have all their inputs and outputs within the distributed filesystem.
In general, there is a need to increase computational efficiency (e.g., increase a number of records processed per unit of given computing resources) of a computation whose underlying specification is in terms of a graph, as compared to approaches described above, in which components (or parallel executing copies of components) are hosted on different servers. Furthermore, it is desirable to be able to adapt to varying computation resources and requirements. There is also a need to provide a computation approach that permits adapting to variation in the computing resources that are available during execution of one or more graph based computations, and/or to variations in the computation load or time variation of load of different components of such computations, for example, due to characteristics of the data being processed. There is also a need to provide a computation approach that is able to efficiently make use of computational resources with different characteristics, for example, using servers that have different numbers of processors per server, different numbers of processor cores per processor, etc., and to support both homogeneous as well as heterogeneous environments efficiently. There is also a desire to make the start-up of graph-based computations quick. One aspect of providing such efficiency and adaptability is providing appropriate separation and abstraction barriers between choices made by a developer at the time of graph creation (at design-time), actions taken by a compiler (at compile-time), and actions taken by the runtime system (at runtime).